Combinatorial receptor codes for odors.
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The discriminatory capacity of the mammalian olfactory system is such that thousands of volatile chemicals are perceived as having distinct odors. Here we used a combination of calcium imaging and single-cell RT-PCR to identify odorant receptors (ORs) for odorants with related structures but varied odors. We found that one OR recognizes multiple odorants and that one odorant is recognized by multiple ORs, but that different odorants are recognized by different combinations of ORs. Thus, the olfactory system uses a combinatorial receptor coding scheme to encode odor identities. Our studies also indicate that slight alterations in an odorant, or a change in its concentration, can change its "code," potentially explaining how such changes can alter perceived odor quality. (+info)
A spatial map of olfactory receptor expression in the Drosophila antenna.
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Insects provide an attractive system for the study of olfactory sensory perception. We have identified a novel family of seven transmembrane domain proteins, encoded by 100 to 200 genes, that is likely to represent the family of Drosophila odorant receptors. Members of this gene family are expressed in topographically defined subpopulations of olfactory sensory neurons in either the antenna or the maxillary palp. Sensory neurons express different complements of receptor genes, such that individual neurons are functionally distinct. The isolation of candidate odorant receptor genes along with a genetic analysis of olfactory-driven behavior in insects may ultimately afford a system to understand the mechanistic link between odor recognition and behavior. (+info)
A PET study of sequential finger movements of varying length in patients with Parkinson's disease.
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To study the difficulty that patients with Parkinson's disease have in performing long sequential movements, we used H2(15)O PET to assess the regional cerebral blood flow (rCBF) associated with the performance of simple repetitive movements, well-learned sequential finger movements of varying length and self-selected movements. Sequential finger movements in the Parkinson's disease patients were associated with an activation pattern similar to that found in normal subjects, but Parkinson's disease patients showed relative overactivity in the precuneus, premotor and parietal cortices. Increasing the complexity of movements resulted in increased rCBF in the premotor and parietal cortices of normal subjects; the Parkinson's disease patients showed greater increases in these same regions and had additional significant increases in the anterior supplementary motor area (SMA)/cingulate. Performance of self-selected movements induced significant activation of the anterior SMA/cingulate in normal subjects but not in Parkinson's disease patients. We conclude that in Parkinson's disease patients more cortical areas are recruited to perform sequential finger movements; this may be the result of increasing corticocortical activity to compensate for striatal dysfunction. (+info)
The neural consequences of conflict between intention and the senses.
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Normal sensorimotor states involve integration of intention, action and sensory feedback. An example is the congruence between motor intention and sensory experience (both proprioceptive and visual) when we move a limb through space. Such goal-directed action necessitates a mechanism that monitors sensorimotor inputs to ensure that motor outputs are congruent with current intentions. Monitoring in this sense is usually implicit and automatic but becomes conscious whenever there is a mismatch between expected and realized sensorimotor states. To investigate how the latter type of monitoring is achieved we conducted three fully factorial functional neuroimaging experiments using PET measures of relative regional cerebral blood flow with healthy volunteers. In the first experiment subjects were asked to perform Luria's bimanual co-ordination task which involves either in-phase (conditions 1 and 3) or out-of-phase (conditions 2 and 4) bimanual movements (factor one), while looking towards their left hand. In half of the conditions (conditions 3 and 4) a mirror was used that altered visual feedback (factor two) by replacing their left hand with the mirror image of their right hand. Hence (in the critical condition 4) subjects saw in-phase movements despite performing out-of-phase movements. This mismatch between intention, proprioception and visual feedback engendered cognitive conflict. The main effect of out-of-phase movements was associated with increased neural activity in posterior parietal cortex (PPC) bilaterally [Brodmann area (BA) 40, extending into BA 7] and dorsolateral prefrontal cortex (DLPFC) bilaterally (BA 9/46). The main effect of the mirror showed increased neural activity in right DLPFC (BA 9/ 46) and right superior PPC (BA 7) only. Analysis of the critical interaction revealed that the mismatch condition led to a specific activation in the right DLPFC alone (BA 9/46). Study 2, using an identical experimental set-up but manipulating visual feedback from the right hand (instead of the left), subsequently demonstrated that this right DLPFC activation was independent of the hand attended. Finally, study 3 removed the motor intentional component by moving the subjects' hand passively, thus engendering a mismatch between proprioception and vision only. Activation in the right lateral prefrontal cortex was now more ventral than in studies 1 or 2 (BA 44/45). A direct comparison of studies 1 and 3 (which both manipulated visual feedback from the left hand) confirmed that a ventral right lateral prefrontal region is primarily activated by discrepancies between signals from sensory systems, while a more dorsal area in right lateral prefrontal cortex is activated when actions must be maintained in the face of a conflict between intention and sensory outcome. (+info)
Localization of the first and second somatosensory areas in the human cerebral cortex with functional MR imaging.
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BACKGROUND AND PURPOSE: Our objective was to map by means of a conventional mid-field (1.0 T) MR imaging system the somatosensory areas activated by unilateral tactile stimulation of the hand, with particular attention to the areas of the ipsilateral hemisphere. METHODS: Single-shot echo-planar T2*-weighted imaging sequences were performed in 12 healthy volunteers to acquire 10 contiguous 7-mm-thick sections parallel to the coronal and axial planes during tactile stimulation of the hand. The stimulation paradigm consisted of brushing the subjects' palm and fingers with a rough sponge at a frequency of about 1 Hz. RESULTS: Stimulation provoked a signal increase (about 2% to 5%) that temporally corresponded to the stimulus in several cortical regions of both hemispheres. Contralaterally, activation foci were in the anterior parietal cortex in an area presumably corresponding to the hand representation zone of the first somatosensory cortex, in the posterior parietal cortex, and in the parietal opercular cortex forming the upper bank of the sylvian sulcus and probably corresponding to the second somatosensory cortex. Activation foci were also observed in the frontal cortex. Ipsilaterally, activated areas were in regions of the posterior parietal and opercular cortices roughly symmetrical to those activated in the contralateral hemisphere. The same activation pattern was observed in all subjects. CONCLUSION: The activated areas of the somatosensory cortex described in the present study corresponded to those reported in other studies with magnetoelectroencephalography, positron emission tomography, and higher-field functional MR imaging. An additional area of activation in the ipsilateral parietal operculum, unnoticed in other functional MR imaging studies, was also observed. (+info)
Segregating the functions of human hippocampus.
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It is now accepted that hippocampal lesions impair episodic memory. However, the precise functional role of the hippocampus in episodic memory remains elusive. Recent functional imaging data implicate the hippocampus in processing novelty, a finding supported by human in vivo recordings and event-related potential studies. Here we measure hippocampal responses to novelty, using functional MRI (fMRI), during an item-learning paradigm generated from an artificial grammar system. During learning, two distinct types of novelty were periodically introduced: perceptual novelty, pertaining to the physical characteristics of stimuli (in this case visual characteristics), and exemplar novelty, reflecting semantic characteristics of stimuli (in this case grammatical status within a rule system). We demonstrate a left anterior hippocampal response to both types of novelty and adaptation of these responses with stimulus familiarity. By contrast to these novelty effects, we also show bilateral posterior hippocampal responses with increasing exemplar familiarity. These results suggest a functional dissociation within the hippocampus with respect to the relative familiarity of study items. Neural responses in anterior hippocampus index generic novelty, whereas posterior hippocampal responses index familiarity to stimuli that have behavioral relevance (i.e., only exemplar familiarity). These findings add to recent evidence for functional segregation within the human hippocampus during learning. (+info)
Optical imaging of functional domains in the cortex of the awake and behaving monkey.
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As demonstrated by anatomical and physiological studies, the cerebral cortex consists of groups of cortical modules, each comprising populations of neurons with similar functional properties. This functional modularity exists in both sensory and association neocortices. However, the role of such cortical modules in perceptual and cognitive behavior is unknown. To aid in the examination of this issue we have applied the high spatial resolution optical imaging methodology to the study of awake, behaving animals. In this paper, we report the optical imaging of orientation domains and blob structures, approximately 100-200 micrometer in size, in visual cortex of the awake and behaving monkey. By overcoming the spatial limitations of other existing imaging methods, optical imaging will permit the study of a wide variety of cortical functions at the columnar level, including motor and cognitive functions traditionally studied with positron-emission tomography or functional MRI techniques. (+info)
The role of area 17 in visual imagery: convergent evidence from PET and rTMS.
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Visual imagery is used in a wide range of mental activities, ranging from memory to reasoning, and also plays a role in perception proper. The contribution of early visual cortex, specifically Area 17, to visual mental imagery was examined by the use of two convergent techniques. In one, subjects closed their eyes during positron emission tomography (PET) while they visualized and compared properties (for example, relative length) of sets of stripes. The results showed that when people perform this task, Area 17 is activated. In the other, repetitive transcranial magnetic stimulation (rTMS) was applied to medial occipital cortex before presentation of the same task. Performance was impaired after rTMS compared with a sham control condition; similar results were obtained when the subjects performed the task by actually looking at the stimuli. In sum, the PET results showed that when patterns of stripes are visualized, Area 17 is activated, and the rTMS results showed that such activation underlies information processing. (+info)